What will it take for Oregon to recover after a magnitude 9.0 Cascadia subduction zone earthquake? Among other things, it will need a major airport to receive the tons of relief supplies from around the world. Prof. Armin Stuedlein and a research team from Oregon State University conducted crucial testing that guided engineers with big plans to make sure one of the runways at Portland International Airport survives the shaking.
STEVE FRANDZEL: A few years ago, a FEMA administrator summed up the aftermath of a Cascadia subduction zone earthquake. This is what he said: “Our operating assumption is that everything west of Interstate 5 will be toast.”
So there’s something about that little sentence that just sticks with me, more than all the ominous facts and figures and warnings. I really can’t explain why. I don’t even know exactly what “being toast” means. But it sure fires up my imagination, and it reminds me that I need to pay attention. That everyone in the hazard zone needs to pay attention and prepare for a big earthquake.
In this episode, we’ll look at some preparation on a massive scale, and the groundbreaking — literally groundbreaking — work by a team of College of Engineering researchers that have shaped those preparations — and saved Oregon tens of millions of dollars in the process. I’m your host, Steve Frandzel.
[MUSIC: “The Ether Bunny,” by Eyes Closed Audio, licensed under CC by 3.0]
FRANDZEL: From the College of Engineering at Oregon State University, this is Engineering Out Loud.
ARMIN STUEDLEIN: Following a rupture of the Cascadia subduction zone, the potential damage to our infrastructure networks can be large. What will really drive just how large these damaged networks will be, will be if the Cascadia zone exhibits a partial rupture or a full rupture.
FRANDZEL: That’s my guest, Armin Stuedlein, a professor of geotechnical engineering at Oregon State. The Cascadia subduction zone is a 600-mile fault that lies about 70 to 100 miles off the Pacific shoreline, and stretches from northern Vancouver Island to northern California.
A partial rupture of the fault will produce a magnitude 8.0 to 8.7 earthquake. If the entire fault ruptures, then the earthquake is expected to exceed magnitude 9.0. FYI, the amount of energy released by a 9.0 is 32 times more than from an 8.0.
STUEDLEIN: What we’re anticipating is that many bridges will be out of service — utilities that supply gas, electricity, potable water, storm water — A lot of these types of utilities will be potentially out of commission.
FRANDZEL: Three quarters of the buildings in Oregon aren’t designed to withstand a megathrust earthquake. FEMA estimates that something like a million structures across the region will collapse or be compromised. And this is all before the rash of secondary disasters, like floods, fires, dam breaches, and hazardous materials spills. Then the tsunami will barrel ashore and take the destruction to another level.
STUEDLEIN: Our recovery really hinges on our ability to restore those critical infrastructure networks, and that’s where the Port of Portland comes in. Having a usable runway immediately following this Cascadia subduction zone earthquake will be critical for bringing in supplies from other parts of the nation and putting our community in a position of responding quickly.
FRANDZEL: The Port of Portland owns and operates Portland International Airport. PDX. Tons of supplies will need to come in round the clock for weeks, maybe months. Food, water, medical supplies, building materials, you name it. The only place in Oregon west of the Cascades that can handle that mission is PDX. But the runways, at least one of them, have to survive. They won’t. They’ll be toast. But that’s going to change.
STUEDLEIN: We’ve got the north runway that’s in proximity to the Columbia River, and then we have the terminals, and then the south runway is located perhaps three quarters to a full kilometer away from the Columbia River. Without mitigation strategies and following an earthquake, the ground supporting the north runway will likely have moved out laterally into the river channel and will have sunk down as a result of that lateral translation.
FRANDZEL: This movement will result from liquefaction, when otherwise solid ground starts acting like a liquid. More about that in a minute.
STUEDLEIN: Essentially, a wide swath of that material will have slumped down under the weight of the non-liquified material, forcing the liquefied material into the river channel. So, we would expect to see potentially portions of the north runway inundated by the Columbia River.
FRANDZEL: What remains above water will be warped and fractured, unusable. And the south runway?
STUEDLEIN: What we’d expect to see is uneven deformations of that runway as it has settled differentially due to the deep liquefaction below it.
FRANDZEL: There are three main soil layers under PDX.
STUEDLEIN: The first layer is approximately a 15-foot-thick hydraulic fill sand material that was dredged from the river bottom along the Columbia River. And it’s really well-known how those soils are going to behave during an earthquake. If saturated, they will certainly liquefy due to their loose nature. The next main soil layer is a deposit of a medium plasticity silt, which won’t liquefy under earthquake loading, but would be potentially subject to large strength loss during the earthquake nonetheless. And from a depth of anywhere about 30 to 40 feet to up to a hundred feet and deeper, we have a deposit of medium-dense liquefiable sand deposits, which is really the deposit of main concern, because that drives the depth to which foundations need to extend or the degree of mitigation, for example, that will be implemented below the runways.
FRANDZEL: So, a little about liquefaction. Liquefaction describes the process of what happens when soil takes on the characteristics of a viscous fluid. The phenomenon is most likely to occur in loose, granular soil that’s saturated with groundwater. Sandy soil, for instance. An earthquake generates fast-moving compression waves and slower-moving shear waves. Vibrations caused by shear waves can trigger liquefaction.
STUEDLEIN: We essentially have water filling the pore spaces in between the individual solid grains, and so what a soil susceptible to liquefaction will experience is that individual grains will want to move relative to one another.
FRANDZEL: Water, however, is nearly incompressible.
STUEDLEIN: And as a result, as those grains move into, or try to move into, the void space that was occupied by water, the water pressure rises. As water pressure rises, it will begin to counteract the gravitational force acting on these soil grains that give rise to its strength. And as a result, we can begin to observe a transition as it becomes a kind of viscous liquid.
FRANDZEL: As the soil weakens, structures built on top of it may warp, sink, or slide sideways. Or all of the above.
STUEDLEIN: So, we’ve got a pretty good idea and understanding of how relatively shallow soils behave under earthquakes, but our case histories are relatively limited for deep soil liquefaction by virtue of being unable to easily observe liquefaction phenomena at that depth.
FRANDZEL: The Port of Portland is developing a plan to strengthen the soil under one of the runways. But first, they needed detailed information about how the ground beneath PDX will behave in the event of a Cascadia subduction zone earthquake. Armin’s team was enlisted to figure it out. To do that, they needed to recreate the subsurface conditions expected during and after a big earthquake by using a technique called blast-induced liquefaction.
STUEDLEIN: Geotechnical engineers have been using, on and off, in a research setting, controlled blasting in order to simulate the raised excess pore pressures — the pore pressures that lead to destabilization of the soils — for some time. In fact, work by other researchers even here at Oregon State University has been focused on trying to understand the consequences of liquefaction through deformations of soils and how foundations interact with liquefied soil during earthquakes.
FRANDZEL: That previous research, however, didn’t capture the relationship between the intensity of the forces generated by an earthquake and the degree of excess pore pressure they caused. Defining that relationship — which had never been done before — was one of the objectives of the PDX work.
The test site was located about a kilometer southeast of the end of the south runway. It was selected because its soil layers were comparable to those below the south runway proper.
STUEDLEIN: In order to make the kinds of observations that we needed to make, we were really required to observe a variety of fundamental, let’s say ground motion characteristics and soil responses. So, the first thing we needed to do was drill deep boreholes to place what we call triaxial geophones.
FRANDZEL: They developed and fabricated a novel geophone sensor package. Forty-two of them were positioned at various depths down four boreholes.
STUEDLEIN: These sensors allow us to pick up the blast-induced ground motions in three orthogonal directions, so we can think of east, west, north, south, and up and down. But because we were essentially timing the passage of seismic waves in order to calculate shear strains and shear stresses, we needed to know exactly where these sensors were with regard to one another within approximately one or two millimeters. The distance was critical. And so, we needed to also develop an instrumentation technique in order to understand exactly how far apart these sensors were. And keep in mind that these sensors were approximately 2.4 meters apart, or eight feet, so one millimeter over 2,400 millimeters is not too shabby in my opinion.
FRANDZEL: Explosives were placed down 16 other boreholes. The charges were selected to release the energy equivalent of a magnitude 7.0 earthquake at a distance of about 10 kilometers from its origin, or a magnitude 8.5 to 9.0 earthquake at a distance of about a hundred kilometers.
STUEDLEIN: And that’s pretty much where our Cascadia subduction zone lies relative to the Port of Portland.
FRANDZEL: To determine the detonation sequence and amount of explosive charge, the researchers turned to some of the previous research that Armin mentioned earlier, including some pivotal work by Scott Ashford, who is now the dean of Oregon State’s College of Engineering. He and his research partner from Brigham Young University, Kyle Rollins, were the first to confirm that blasting could be used to induce liquefaction when testing ground improvements and deep foundations. But the goal of the PDX research was different. It was to determine the fundamental dynamic response of soil during and after an earthquake: What, exactly, was going on underground? That was also a first.
The tests were conducted over three days, starting on October 3rd, 2018.
STUEDLEIN: We were able to get a 360-degree view of how liquefaction is triggered at very large depths and what the corresponding consequences in terms of displacement would be. And the soil responds in the same manner to these blast-induced shear waves as they would to an earthquake. And so that was one of the main outcomes of our work is to demonstrate that this technique is viable in terms of relating the earthquake performance of our soils, and that we can indeed develop fairly complex soil dynamics, soil properties, from our experimental technique, which were then used directly by the port’s engineers to design their mitigation strategy that they’ll use below that south runway.
FRANDZEL: Port engineers were already leaning toward a technique called deep-soil mixing to strengthen the ground below the runway. It involves injecting water and cement slurry into the soil. The mixture changes the composition of the ground into a kind of artificial sandstone, which isn’t subject to liquefaction.
STUEDLEIN: I believe that the results of our work essentially confirmed that that was going to be the strategy that they were going to move forward with.
FRANDZEL: But the data also told another story: Without the benefit of the blasting experiments, the engineers at PDX would probably have applied deep-soil mixing to a depth of about a hundred feet. But the research told them they didn’t need to.
STUEDLEIN: With the site-specific and at-depth measurements, they were able to show that less mitigation depth would be necessary to achieve the performance requirements that they wanted for their south runway.
FRANDZEL: In fact, based on the test results, deep-soil mixing only had to reach down 50 feet to yield sufficient runway performance. At a cost of about $1 million for every additional foot, that meant saving a lot of money.
STEUEDLEIN: Yeah, it appears that the difference between 50 and a hundred feet may have been about $35 to $50 million worth of construction.
FRANDZEL: The Port of Portland contributed $350,000 toward the cost of the project.
STEUEDLEIN: That puts the rate of return, or the return on investment, of about 50 to a hundred to one.
FRANDZEL: The Port of Portland engineers have completed a third of the design work to prepare for ground improvements. When they’re done with the initial phase of design, they’ll seek federal funds and, hopefully, start deep-soil mixing under a 6,000-foot stretch of the south runway within 10 years. Similar tests at the port of Longview along the Washington side of the Columbia River produced results that were just as precise and just as revealing — essentially validating their PDX experiments.
STUEDLEIN: One of the benefits of our controlled blasting test technique is that we can test soils at any depth. So, for example, the mobile shakers that are available for seismic testing tend to be limited to depths of about 10 feet, but we’ve gone as far as depths of about 90 feet at the Port of Portland. The other thing is that we can test soils that are not easily sampled. I think that’s one of the main benefits of our work.
We do hope to continue this work a little bit further. And one of the main interests that we currently have is to test gravelly soils for soil liquefaction, which has been observed in the field following earthquakes. But these kinds of large particles simply do not wish to be sampled. And so, we think that there’s a really good application for our test technique on these kinds of soils moving forward.
FRANDZEL: This episode was produced and hosted by me, Steve Frandzel, with additional audio editing by Will Havnaer. Other members of Armin’s research team were Amalesh Jana, Aleyna Donaldson, and James Batti. Additional funding for the airport tests came from the Cascadia Lifelines Program, a collaboration of academia, government, and industry established at Oregon State. Its director, Michael Olsen, is a professor of geomatics at the university. To find out what you can do to prepare for a major earthquake, and other disasters, check out the links in the show notes. There are some easy steps you can start right away.
Our intro music is “The Ether Bunny” by Eyes Closed Audio on SoundCloud and used with permission of a Creative Commons attribution license. Other music and effects in this episode were also used with appropriate licenses. You can find the links on our website. For more episodes, visit engineeringoutloud.oregonstate.edu, or subscribe by searching “Engineering Out Loud” on your favorite podcast app. Bye now.